Add info event to propagate protocol-level events

[palkan]

Context

Our extended protocol supports automatic stream history retrieval but we do not provide any APIs to react on the corresponding events (more precisely, protocol messages: confirm_history and reject_history).

This PR introduces a new info event for cables and channels that can be used to signal arbitrary protocol-level events to users, including history_not_found and history_received events:

import { createCable, Channel } from '@anycable/web'

const cable = createCable({protocol: 'actioncable-v1-ext-json'});

const ch = cable.subscribeTo("ChatChannel");

ch.on("info", (evt) => {
  if (evt.type === "history_not_found") {
    // Restore state by performing an action
    ch.perform("resetState")
    
    // Or hard-reset the clients by reloading the page
    window.location.reload();
  }

  // Successful history retrieval is also notified
  if (evt.type === "history_received") {
    // ...
  }
});

[cmdoptesc]

Thanks for this speedy enhancement!

[cmdoptesc]

Is it safe to assume this can actually just be:

Suggested change

	type: string
	type: 'info'

[palkan]

It's not 'info', but a protocol-level event ('history_received' or 'history_not_found').

The idea is that we allow the protocol to propagate anything up to the cable and channels, so the possible event types are not known beforehand.

We can probably add some generics and TS magic to make the InfoEvent.typed inferred from the Cable.protocol, but... let's leave it as a potential contribution for some TS enthusiast :)

[cmdoptesc]

And here, could we possibly use a string union type to be more specific?

Suggested change

	type: string
	type: 'history_received' | 'history_not_found'